We also provide a formal policy on metric which is captured here, this represents the formal definition of CCU, Nodes and cores. Cloudera Licensed Metrics
Cloudera Compute Unit (CCU) The link above defines in some detail what variable compute and storage is and the use of the CCU term. I will not duplicate those details here. CCU is simply a unit of measure and I will defer to the updated document as the unit of measure definition can change as pricing policies change.
Generally, we determine the number of nodes based on the count returned in Cloudera Manager. Under Administration > License, the number of nodes is listed and all nodes counted need to be covered by the license.